Earlier quoted context omitted.
In other words, does a16z ask for startups to attack MS on their home turf? There can be a niche for the product creation stage. But how does a company scale in MS's market? Sooner or later, it's a Slack vs MS Teams situation.
To be fair, slack kinda messed up themselves. I am still astonished that they never built video calling. Like MS did use their existing sales channels to stop slack growing, but if slack at least had feature parity with Teams then it would have been a harder sell.
(ETA: Of course, easy to spin things the other direction and read Microsoft's offer as a bully move "join us or we'll build a better you without you". The reality is somewhere in the middle of the two extreme perspectives.)